This question comes up frequently, and Pete Mikalajunas posted these illustrations
Both were trade names used by Henri Pieper to discribe his patented method of joining barrels to the breech monoblochttp://www.littlegun.be/arme%20belge/ar ... i%20gb.htm
From the 1881 Paris ExpositionDiana
with a step from the steel breech to the damascus barrelsModified Diana
without the step, and with a modified lock-up and rib extension. Note the "Washington" pattern damascus barrels with steel breech
1897 Sears catalog - note the warning regarding cheap imitations.
"Diana Style" or "Diana Pattern" were used by lots of makers